The present invention relates to a digital signal coding/decoding circuit and particularly to a preparation circuit for coding/decoding of a digital signal, such as a two level (white and black) facsimile signal.
Heretofore, two coding/decoding systems have been recommended by CCITT for efficient transmission of facsimile signals; one is the MH (Modified Huffman) system making 1-dimensional coding/decoding and the other is the MR (Modified Read) system with 2-dimensional coding/decoding. The latter is predominantly used because of its higher transmission efficiency. The MR coding/decoding requires a memory means for storing two adjacent scanning lines, that is, a scanning line to be coded or decoded and a previous scanning line which has been previously coded or decoded. To hold the previous scanning line, a line buffer is used for temporarily storing the previous scanning line. The stored scanning line is used as a reference line for coding or decoding a successive scanning line.
Further, if an image to be transferred is scanned with high-resolution, the amount of digital information per line increases. Therefore, a memory with a large capacity is required as the line buffer. For example, in the case of sixteen scanning signals per 1 mm, 3456 bits per one line must be coded for A4 size paper. In practical use, the reading operation of the line used as a reference line at a certain time is performed at substantially the same time as the writing into buffer operation of the next line to be used as the reference line. Two lines of buffer memory are then required in the line buffer. Therefore, a conventional coding/decoding circuit needs a line buffer with a large capacity. Consequently, it is hard to form a one-chip semiconductor integrated circuit containing the line buffer together with a coding/decoding unit.